This invention relates to radio frequency interference (RFI) chokes designed to impede the flow of spurious RF currents along the outside of an outer coaxial cable conductor. The invention has particular application to an improved magnetic resonance imaging (MRI) system where the coaxial cable RFI choke assembly is employed in connection with at least one RF coaxial cable transmission line utilized to couple MRI processing circuits with a remotely located RF coil assembly (e.g., located in a nuclear magnetic resonance polarizing magnet, magnetic gradient coils and the like).
The problem of spurious RF currents conducted on the outside of MRI RF coaxial cable transmission lines has been previously noted and addressed in related commonly assigned U.S. Pat. No. 4,682,125, naming Harrison et al as inventors and entitled "RF Coil Coupling for MRI with Tuned RF Rejection Circuit Using Coax Shield Choke" (see FIGS. 1 and 2A). It is also believed to have been addressed by earlier commercial devices (e.g., the Toshiba "QD Brain Coil" marketed in at least Japan for more than one year) which have used a factory adjusted tuning capacitor (instead of a movable conductor core) for a single channel RF breaker circuit box affixed to an MRI RF coil. It appears from available text to be connected to printed circuit board ground potential and it is hermetically encased in a conductive housing box approximately 2.times.1.25.times.1.5 inches in dimension. A separate such breaker is used for each QD coil channel and they are separated by about 20 centimeters. Coaxial connectors are mounted to the hermetically sealed conductive box (see FIG. 2B).
In the earlier Harrison et al prior art, an MRI RF choke is realized by forming a coiled section of flexible coaxial cable transmission line with a lumped and fixed capacitance connected to the coaxial braid in parallel across the coil. A conductive tuning rod is positioned within the center of the coiled coaxial cable section so as to trim the parallel resonant frequency to a desired value. The exemplary embodiment described by Harrison et al resulted in a single channel RF choke assembly approximately six inches long and about 1.5 inches in outside diameter. Such a bulky RFI assembly is poorly suited for location directly in or on an MRI RF coil. Interconnecting RF cables are clumsy at best and easily damaged by mishandling during use or when changing RF coil assemblies (e.g., typically requiring unscrewing of a dangling bulky unit).
Even if the physical size of the prior art RF ground breaker is straightforwardly reduced as much as possible, it is still too large for optimum convenient use and there are still other problems as well. For example, the exemplary Harrison et al embodiment was tuned by moving a conductive rod inside the inductor and this is sometimes difficult for use with specific coils. Furthermore, the coaxial cables emanating from the MRI RF coils often have to be bunched together for acceptable operation and/or service. Since the relatively large Harrison et al type RF ground breakers are typically located at least a short distance away from the RF coil itself with transmission lines to the RF coils at the image volume, this bunching of cables often leads to degradation of the quality factor Q associated with the parallel resonant tuned RF ground breaker circuits. Thus, the desired RF "ground breaking" isolation function is itself impaired.
Although not as much is known about other commercially available capacitively-tuned MRI RF ground breakers, observation of the Toshiba QD Brain Coil single channel breakers has disclosed that they are widely separated from one another in the RF coil apparatus and further isolated by individual hermetically sealed conductively shielded housings.
These problems have led us to a new more compact and efficient RF ground breaker design for MRI applications. For example, in our new design, we incorporate a variable capacitor to more simply and reliably tune the parallel resonant ground breaker circuit to the desired frequency range. Furthermore, by making the ground breaker assembly small enough to actually fit directly on or inside the RF coil (e.g., inside a "QD" or quadrature-detection head coil), bunched RF transmission lines occur only downstream of the RFI choke assembly thus avoiding Q degradation problems.
Since many MRI RF coil structures actually comprise a plurality of coils (e.g., a pair of quadrature-detection coils in a typical head coil), our preferred embodiment actually constitutes a double ground breaker formed on a single printed circuit substrate. Undesirable coupling between the closely proximate ground breaker assemblies is neutralized or cancelled by appropriate inter-channel capacitance of coupling. There is no necessity for elaborate hermetic shielding or the like.
Whether our single or double (or more) channel RF ground breakers are used, they are preferably formed on a single printed circuit board (PCB). In addition, the printed circuit board structure is preferably double sided so that some of the ground breaker circuit components may be located on both sides of the printed circuit board structure. In the preferred exemplary embodiments, standard RF coaxial cable connectors are mounted (in PCB recesses) to printed circuit conductive traces on both sides of the printed circuit board (thus serving to assist in interconnection of both sides of the printed circuit board while at the same time providing a compact and robust physical mounting for an RF connector).
We have devised several exemplary types of single channel ground breakers. A first straight single channel type has RF coaxial connectors disposed at opposite sides or ends of the printed circuit board structure. A second "bent" single channel type has RF connectors projecting from the same side of the printed circuit board. For the most part, they perform equally well although in some cases, it has been observed that the bent single ground breaker may actually perform better. It has been discovered that the Q quality factor of an MRI RF coil using these new ground breakers is better than when the prior art ground breakers are utilized. Furthermore, the newer ground breakers of this invention exhibit a much lower insertion loss than the prior art ground breakers. For example, the newer ground breakers have exhibited insertion losses on the order of about 0.070 dB, while prior art ground breakers exhibit insertion losses on the order of 0.110 dB to 0.141 dB.
As will be appreciated by those in the art, a relatively high voltage rated capacitor should be used for MRI applications. For example, it is estimated that the capacitors utilized should be capable of withstanding at least 600 volts for a 2 kilowatt RF MRI circuit. Of course, such voltage requirements can be met by either utilizing a single high voltage rated capacitor or a plurality of lower voltage rated capacitors connected in series. When series connected variable capacitors of approximately equal voltage ratings are utilized, they should be adjusted approximately by equal amounts so as to maintain the divided voltage across the two capacitors as uniform as possible.
To reduce interference between the two channels of a "double" (or more channel) ground breaker, a neutralizing capacitor (or capacitors) is coupled between the ground breakers.
The MRI RFI ground breakers of this invention are physically so small and compact that they can be located on or inside an MRI RF coil. They are easily manufactured, tuned and handled. Two or more ground breakers can be located in a very small space. They are also very stable in performance and can be fabricated using PCB in common with the MRI RF coil itself and without the need for elaborate hermetic shielding.